Why does a Main Sequence star eventually grow in size of a Red Giant?

Explanation:

When stars first begin to fuse hydrogen to helium, they lie on the zero-age main sequence. ... The core of a red giant is contracting, but the outer layers are expanding as a result of hydrogen fusion in a shell outside the core. The star gets larger, redder, and more luminous as it expands and cools.

b. the carbon cycle ensures living things have a source of hydrogen

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